Optical sensor is often used for counting the number of banknotes in an automatic teller machine (ATM) or the like, for example. FIG. 9 schematically illustrates a method of detecting a banknote in a conventional ATM. As illustrated in FIG. 9(A), in a conventional method, a banknote is detected when the LED light is interrupted by the banknote. This method is widely used in general because presence or absence of a banknote can be detected in a contactless manner. However, a trouble occurs in the case of a foreign banknote as illustrated in FIG. 9(C).
That is, regarding a foreign banknote, a Polymer banknote is used recently in order to enhance durability. In the case of a design-conscious banknote as illustrated in FIG. 9(C), there is a transparent portion in the banknote whereby a problem of misdetection occurs. In other words, since a transparent portion transmits light from an LED, it may be erroneously recognized that there is no banknote.
As a method of preventing such misdetection, a resistance measuring method is known (see FIG. 9(B)). In this method, electrodes are brought into contact with each other, and by allowing a banknote to pass between the contacting electrodes, interruption of a connection between the electrodes is detected. In this method, even in the case of a banknote with a transparent portion as described above, presence or absence thereof can be detected reliably. However, since a banknote must be in contact with electrodes, there is a problem that the banknote is damaged.
Further, as a technology of detecting banknote fatigue with use of ultrasonic waves, there is a technology disclosed in Patent Literature 1, for example. The technology disclosed in Patent Literature 1 is a medium fatigue detection device including an ultrasonic wave transmission element (11) that oscillates ultrasonic waves as burst waves, an ultrasonic wave receiving element (12), and a receiver sensitivity detection unit (22). The elements (11) and (12) are arranged at positions opposite to each other via a sheet-like medium (5) such as a banknote passing between the both elements. Ultrasonic waves transmitted from the element (11) are made incident on the main surface of the sheet-like medium (5) at a predetermined incident angle θ. By measuring the variation in the receiver sensitivity of the ultrasonic waves passing through the sheet-like medium (5), fatigue of the medium (5) is detected.